Mexico became the first country in the world to allow voters to elect judges at all levels on 11 September.
The decision came after protesters stormed the Upper House and debate on the issue was postponed.
Outgoing President Andrés Manuel López Obrador had pushed hard for reform and criticized the current judicial system for serving the interests of political and economic elites.
With 41 votes against and 86 in favour, the reform was approved by the upper chamber, which is dominated by the ruling MORENA party and its allies.
This is the two-thirds majority needed to amend the constitution.
The debate over reform has sparked mass protests, diplomatic tensions and investor panic.
Opponents, including court employees and law students, have staged numerous protests against the plan, which would see Supreme Court and other high-level judges, as well as local-level judges, elected by popular vote.
